Hi, I used subsurface modifier, and as you can see the end of the shirt, everything is sharp except for the front ones. I just tried everything to make them sharp instead of being curved. I don't know what is wrong..
[duplicate] You have bad normals all over the place, that's what those odd dark shadings are. Select all verts in edit mode, control n to recalculate normals.
It looks like that there are some verticies between the body and the shirt. Therfore the "front ones" are not the last ones, so it bends inside.